This invention relates to a bandpass filter for a community antenna television (CATV) converter.
Reference being made to FIG. 4 showing an example of bandpass filter which was previously designed for a CATV converter during the course of research leading to the present invention, the bandpass filter 30 comprises a constant-K low-pass filter section 33 wherein inductors L.sub.11 -L.sub.15 and capacitors C.sub.11 -C.sub.14 are longitudinally connected in a plurality of stages and a high-pass filter section 34 composed of a constant-K filter with an inductor L.sub.16 and a capacitor C.sub.15 and an inductive M-type filter with an inductor L.sub.17 connected between an input terminal 31 and an output terminal 32. The low-pass filter section 33 may be designed, for example, to have a pass zone of 0-450 MHz, a damping zone 470 MHz and over and a cutoff frequency 470 MHz. The high-pass filter section 34 may likewise be designed to have a damping zone 50 MHz and below and a cutoff frequency 50 MHz.
In the United States, tuners are tested under the condition that 64 wave signals mutually separated by 6 MHz are present at a fixed input level (-6 dB.sub.mv to +15 dB.sub.mv) within the range from 55.25 MHz to 445.25 MHz. It is generally considered undesirable to input a signal to an active element at an input level of +15 dB.sub.mv or greater because of the occurrence of intermodulations, crossmodulations, backtalks, local leakages, spurious responses at the input terminals, etc. When a bandpass filter of the type shown in FIG. 4 is used, therefore, a high-capacity mixer such as a double-balance mixer is used at a later stage. But the double-balance mixer is in the form of a bridge with four paired diodes, and since it requires a balloon transformer with good phase balancing, it cannot be mass-produced easily. Moreover, an empirical procedure and balance adjustments are required for maintaining a spurious response level of -60dB or greater.